This invention relates generally to electrical metering, and in particular to providing a portable apparatus for metering.
The manner in which electrical power is generated and distributed is generally of no concern to a consumer who has simply come to expect that electrical power will always be available. The distribution of electrical power, however, is not a simple task. On a basic level, a power system takes electrical power from a generator, such as a steam or hydraulic turbine, and transports the electrical power over a distribution network to the customers"" premises. On a more detailed level, the distribution network is a network of lines, often both overhead and underground, and includes transmission, sub-transmission, and distribution lines. The voltages on the lines are stepped up at the generator, and then progressively stepped down or reduced as the electrical power travels from the transmission lines, to the sub-transmission lines, and then to the distribution lines. The distribution lines then carry the electrical power to the customers"" premises, where the electrical power is typically stepped down once more before being transported to the customers"" wiring.
In addition to the lines and generators, a power distribution network also includes a number of transformers dispersed throughout the network. In general, a transformer is a device that takes electrical power having one voltage and outputs the same electrical power at a different voltage. A transformer has a set of primary windings or coils which are coupled to a set of secondary windings or coils, with the primary and secondary windings often being immersed in oil. Based on a ratio of the number of primary windings to the number of secondary windings, the voltage can be stepped up to a higher voltage or stepped down to a lower voltage as the electrical power passes from the primary windings to the secondary windings. The transformers are used extensively throughout the network and couple generators to the transmission lines, step down the voltages between the transmission lines and sub-transmission lines, step-down voltages between the sub-transmission lines and the distribution lines, and finally step-down voltages between the distribution lines and the customer premises.
For electrical power to be distributed by local utilities or utilized by large industrial customers, transmission voltages, typically, greater than 25 kV, must be stepped down for introduction into a distribution grid. Typically, this voltage reduction is accomplished by a substation transformer. Occasionally, these transformers must be replaced or repaired. During this time, the defective transformer must be disconnected from the power network so that the necessary work can be safely performed. Frequently, an electrical revenue meter and associated instrument transformers are located in close proximity to the transformer so when the defective transformer is disconnected from the network the electrical revenue meter and associated instrument transformers are necessarily also disconnected. As a result, the electrical revenue meter and associated instrument transformers are unable to measure the amount of power that is being provided through a temporary hook-up, such as through a temporary transformer. Because the electrical revenue meter and associated instrument transformers are inoperable during repairs and replacement of a transformer, the amount of power being delivered through the network is often estimated based on historical usages or other such averages. For example, the amount of power used for the same period last year may form the basis for the estimate of power delivered during the work period. The estimates of power usage are subject to a great degree of inaccuracy, especially in light of inevitable weather and load variations. Inaccurate estimates typically translate into over- or under-recovery of revenue from the sale of electric power. This estimate is often an unsatisfactory measure of the power and may be commercially unacceptable to the interested entities.
Electrical power, as with many other utilities, is being deregulated or has been deregulated in many states. The price for power no longer remains relatively constant year-by-year but instead can fluctuate day-by-day or even hour-by-hour. The fluctuations and volatility in electric power pricing is most evident in the electrical power crisis that affected California in 2001 and continues to plague California as a result of deregulation. California experienced escalating prices for electrical power and demand which exceeded the capacity of the power grid. Consumers took drastic measures to reduce their consumption of electrical power and experienced black-outs, rolling black-outs, and brown-outs. The price of electrical power is therefore no longer a routine fixed cost.
Because of deregulation and also other market conditions, more accurate metering of electrical power is being mandated while work is being performed on the network. As mentioned above, the metering associated with a transformer is so closely associated with the transformer that it effectively cannot be used during the work period. However, temporary metering can be accomplished by bypassing the permanent substation meters and substituting a temporary metering station.
Installing the temporary metering station typically involves a labor-intensive, time-consuming, and therefore, expensive process. Installation usually requires a first work crew, using the auger of a digger derrick or similar earth moving equipment, to dig an approximately 10xe2x80x2 deep hole. Using a distribution crane, a second work crew lifts a distribution power pole and xe2x80x9csetsxe2x80x9d the pole in the hole, which is then backfilled with dirt. Next, an xe2x80x9cinstrument clusterxe2x80x9d is constructed. For each phase, the instrument cluster includes two instrument transformers. Typically, one of the instrument transformers is a current transformer (CT) for reducing the current, and the other is a potential transformer (PT) for further reducing the voltage to levels that are suitable for measurement using meter instruments. Using a hydraulic tool, the distribution power pole is bored such that the instrument cluster can be mounted thereon. For each phase, connections are made between the secondary side of the substation transformer and each CT and PT. A third crew typically uses a bucket truck to mount the instrument cluster and make the connections. Secondary conductors are buried, slack-spanned, or tension-spanned from the substation transformer to the instrument cluster, and from the instrument cluster to the other substation equipment. Metering conductors are connected at one end to the CT and the PT, and the metering conductors are enclosed in a conduit that is attached down the length of the pole. The other ends of the metering conductors are attached to metering instruments. Only after this work is completed can the temporary substation transformer be energized and the power usage measured by the electrical meter. As can be appreciated by those skilled in the art, the entire process is a custom installation that may take dozens of man-hours to complete, requires the coordination between several work crews, and is a costly endeavor that can easily exceed $10,000.
The work involved in placing a meter is a fraction of the work involved in replacing a defective transformer. As mentioned above, when a defective transformer is being repaired or replaced, a temporary transformer can be brought to the premises to fulfill the functions of the defective transformer while the work is being performed. When the defective transformer is a substation transformer, this temporary transformer is transported to the work site with a tractor-trailer, and is commonly referred to as a xe2x80x9cmobile substation.xe2x80x9d U.S. Pat. No. 4,367,512 to Fujita, U.S. Pat. No. 4,427,898 to Miyake et al., U.S. Pat. No. 4,562,360 to Fujimoto, and U.S. Pat. No. 6,272,733 to Baker, Jr., all of which are incorporated herein by reference, describe temporary portable transformers that may be brought to a work site during the change-out of a defective transformer. As described in these patents, the temporary transformer is connected to the lines to provide the necessary connections previously provided by the defective transformer. Once a new transformer is installed and connected, this temporary transformer is removed. These temporary mobile transformers greatly simply the process of replacing a defective transformer while at the same time maintaining the distribution of power.
One challenge in performing any type of repair within the power network, but which is especially true in replacing a transformer in a substation, is working within the constraints of available space. Often, the substations are dispersed throughout the countryside, at times in remote locations and in fenced-in locations that offer only very limited maneuverability. The rough terrain and limited space can make it difficult for work crews and the tractor-trailers to even access the work site, let alone to move around the site to perform the necessary repairs.
A need therefore exists for improved systems and methods for use when performing work on a power network. A need also exists for systems and methods for use in measuring power delivered while work is performed on the power network that are less costly, more efficient, and which offer a practical solution given the space constraints within the power network.
The present invention addresses the needs identified above by providing systems and methods for monitoring and metering power distribution parameters for a number of purposes at a variety of locations within a power transmission or distribution network. The systems are self-contained and easily transportable to a metering point on an electrical power transmission or distribution network. At the metering point, the systems are adapted to receive input conductors from a source, to transform parameters related to the input conductors, and to provide the transformed parameters to a metering instrument. The systems are also adapted to easily connect the input (source) conductors to output (load) conductors that are connected to a power network.
According to one embodiment, the system comprises a pre-assembled mobile metering system (MMS) for temporary metering. The MMS includes an enclosed pre-wired three-phase metering apparatus. The enclosure protects interior components from atmospheric conditions, and provides easy access to the components when the MMS is de-energized. The parameters monitored by the metering apparatus include current and voltage. The MMS receives high voltage inputs by connecting the secondary side conductors and neutrals of a movable or fixed substation transformer to power bushings and neutral terminals mounted on exterior of the MMS. The MMS transforms the inputs using enclosed metering instruments such as current transformers and potential transformers, and provides meterable currents and voltages that accurately reflect the values of the high voltage inputs. The meterable currents and voltages are safely accessed via an external metering cabinet.
The MMS provides much greater portability than conventional approaches to metering and monitoring power parameters. The MMS is mounted on a trailer that is no more than approximately 16 feet long, such that its total size is compatible with over-the-road highway transportation without the need for large towing vehicles. The MMS is also maneuverable in or around a typical electrical substation yard, so as to gain access to pre-existing system elements, such as conductors leading from a substation transformer, mobile substation, load bus, or transmission line. In contrast, conventional approaches involve the transportation of components that are assembled on site in a process that involves several large vehicles performing the different tasks mentioned above. When the conventional system is no longer needed, the conventional system is disassembled and transported away from the metering point.
Another benefit of this invention is the relative ease of connecting and disconnecting the MMS to conductors and metering devices. The MMS has a reinforced roof that allows substation personnel to connect conductors and neutrals to the MMS while standing on the roof of the MMS enclosure. Therefore, there is no need to provide an additional aerial work surface, such as the bucket of a utility line truck. There is also no need for multiple work crews that are assigned to different types of utility vehicles. For example, a conventional approach may require a two-man crew to drive to the metering site to operate a digging tool, and a separate two-man crew to drive to the metering site to operate a crane. A third three-man crew is also required to drive a bucket truck to the metering site. One member of the bucket crew operates the bucket controls, a second member stands in the bucket and makes connections on the pole, and a third member of the bucket crew provides ground support by handing tools and materials to the second member. Using the MMS, at most, a two-man crew is needed to drive a tow vehicle to the metering point, to place tools and materials on top of the MMS, and to make connections to the MMS. Thus, the MMS can reduce the time taken to complete a metering installation from 1 day to as little as 30 minutes. Additionally, any necessary repairs, component replacements, or adjustments may be performed by simply de-energizing the MMS and accessing any component, without the need for a bucket truck. The outputs from the MMS to a metering instrument can also be easily accessed from the ground via a metering interface cabinet and a conduit that is integral to the load side of the MMS.
Another benefit of the MMS is the ability to utilize either an integral or an independent metering device. For instance, an electrical revenue meter can be permanently installed xe2x80x9conboardxe2x80x9d the MMS. Alternatively, an external or removable revenue meter can be linked to the MMS, using for example, a 10-pole knife switch. This feature facilitates the uses of a variety of types of electrical revenue meters according to the requirements of each particular metering instance. For example, in one instance, a meter with remote communications capabilities may be desirable, while in another instance, remote communication may be unnecessary.
The MMS provides a variety of safety-oriented protective devices, such as an electromagnetic lock that prevents the interior of the MMS from being accessed while any interior components are energized. This feature functions to prevent serious injury or death to personnel, and damage to or theft of internal components. An integral grounding system is also provided, including an external ground grid that is mounted on the roof of the MMS, and an internal ground loop. In contrast, conventional approaches involve energizing components that are fully exposed, and thus subject to accidents and tampering. The integral grounding system creates a continuous ground path by interconnecting the neutral of the mobile substation, the external ground grid, the internal ground loop, and the buried ground plane of the fixed substation.
The MMS also provides compatibility with more than one transmission or distribution voltage. For example, the MMS can be dual or triple voltage rated, such that two or three different transmission voltage levels can be safely and accurately measured. Therefore, the need for multiple trailers for monitoring and metering a variety of metering points can be substantially reduced, or eliminated altogether.
The MMS may contain additional circuit components, such as switchgear, a three-phase recloser and/or a three-phase circuit breaker. The MMS may contain additional communications components and cabling, such as a remote terminal unit (RTU) that allows measurements to be made and control signals transmitted remotely. The MMS can also contain a station service transformer that provides AC power to the substation to maintain battery chargers, and to power outlets and lighting.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become more apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention.